It runs as another program MsMpEng, in the background. Just make sure to check "show processes from all users". In reality is uses around 40 MB of RAM when it is inactive. It still does seem lighter than most AV programs. Whether it works well in the long run, we will see. It has been known to flag ok sofware as malware. (I had seen it do that with quicktime (I know, maybe it was being proactive)